1. Field of the Invention
The present invention relates in general to distance measuring equipment, and more particularly to distance measuring equipment of a laser-radar type. More specifically, the present invention is concerned with laser-radar type distance measuring equipment which is mounted on a motor vehicle to measure the distance from the vehicle to another vehicle running in front, by using the laser beam.
2. Description of the Prior Art
For safety, some modernized motor vehicles are equipped with a distance measuring equipment of the above-mentioned type. That is, when a moving vehicle comes very close to a vehicle in front of it, the equipment gives a visual alarm, an audio alarm or the like to the driver letting him or her know an abnormal approach of the vehicle to the front vehicle. However, due to their inherent constructions, some of them have failed to exhibit a satisfactory performance.
In order to clarify the task of the present invention, two such conventional distance measuring equipment will be outlined with reference to FIGS. 14 to 19 of the accompanying drawings.
Referring to FIGS. 14 to 16, particularly FIG. 14, there is shown a first conventional probe unit 100, which is to be mounted to a front fixed part of a motor vehicle (not shown) with its front part facing forward.
The probe unit 100 comprises generally a housing 101 in which a laser emitting device 102 and a reflected laser receiving device 104 are abreast installed. The housing 101 is mounted on a base plate 110 through left and right brackets 106 and 108. The left bracket 106 is shown in more detail in FIG. 15. Information detected by the probe unit 100 is transmitted through a cable 112 to a control unit (not shown) mounted in the vehicle. The right bracket 108 is incorporated with a position adjuster which includes two position adjusting screws 114 and 116. Thus, the angular position of the probe unit 100 relative to the base plate 110 is adjustable by manipulating the screws 114 and 116. That is, by turning these screws 114 and 116, horizontal and vertical positions of the unit 100 can be adjusted respectively.
Upon requirement of assembly of the probe unit 110 on a vehicle, the base plate 110 is at first secured to the front fixed part of the vehicle by means of bolts or the like. Then, the screws 114 and 116 are manipulated to adjust the angular position of the probe unit 100 (more specifically, the housing 101) relative to the vehicle body. With these steps, the probe unit 100 can be tightly mounted to the vehicle having the laser emitting and receiving devices 102 and 104 directed to a fixed direction.
However, this first conventional distance measuring equipment has such a drawback that when, as is seen from a beam illustrated by a solid line in FIG. 16, the vehicle is traveling along a curve, the laser beam emitted from the laser emitting device 102 fails to make an exact hit on the front vehicle and thus the laser receiving device 104 can not receive the reflected laser sufficiently. Thus, in this case, the distance between the two vehicles can not be measured exactly by the equipment.
Referring to FIGS. 17 to 19, particularly FIG. 17, there is shown the second conventional equipment. For solving the drawback of the above-mentioned first conventional equipment, the second conventional equipment uses a plurality of laser beams (viz., three in the illustrated example) which are emitted forwardly but in different directions as shown in FIG. 18. Thus, at least one of the laser beams has a chance to make an exact hit on the front vehicle.
The equipment comprises a probe unit 100 mounted to the vehicle body in the above-mentioned manner. The probe unit 100 has in its housing 101 a laser emitting device 102 and a reflected laser receiving device 104. Information detected by the probe unit 100 is transmitted to a control unit 200 through a cable 202. Information from a vehicle speed sensor 204 and an ignition switch 206 is also fed to the control unit 200 through respective cables 208 and 210. The probe unit 100 and the control unit 200 are powered by a battery 212.
As is seen from the drawings, the laser emitting device 102 of the probe unit 100 is constructed to issue three pulsed laser beams L, M and R. Counting the time elapsed from the time when a certain pulsed laser beam L, M or R is emitted from the laser emitting device 102 to the time when a part of the laser beam reflected back by a front vehicle is received by the laser receiving device 104, the control unit 200 calculates the distance between the two vehicles.
The vehicle speed sensor 204 is used for increasing the intensity of the laser beams L, M and R in accordance with increase of the vehicle speed. As is known, increasing the intensity of the laser beams L, M and R increases the range of the beams, so that when the vehicle is running at a higher speed, the alarm is issued at relatively early stage, that is, the alarm is given at the time when the distance between the two vehicles is still large. While, when the vehicle is running at a lower speed, such alarm distance becomes smaller than that given at the higher speed running.
However, even this second conventional distance measuring equipment has the following drawback.
Sometimes, the alarm is issued even in a case wherein such alarm is not really necessary. As is seen from FIG. 19, when, during cruising on a curved road, the left laser beam L happens to make a hit on a tree T or the like standing by the road, the equipment gives an unnecessary alarm to the driver irrespective of that fact that a sufficient distance remains between the two vehicles.